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Poster: johnnyonthespot Date: Sep 15, 2009 12:39pm
Forum: GratefulDead Subject: Re: Edwin H. Armstrong and The Grateful Dead

can you answer if it's true what the band claims of 11-7-71 Harding Theater about the sound reaching the aud through the radio faster than in the hall? I have a hard time buying it.

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Poster: dead-head_Monte Date: Sep 5, 2010 10:33am
Forum: GratefulDead Subject: Re: Edwin H. Armstrong and The Grateful Dead

I finally d/l the Harding Theatre SBD yesterday. Today I listened to the Intro track. Jer says, "If you're sitting at home next to your radio, you're hearing the music faster than you are if you're in the hall. That's an interesting but true..." Bob says, "We won't bore you with lengthy explanations of this phenomenon." Then they shake out the last feedback tones in the PA system and the monitors. The sound crew are setting the equalizers, mix levels, and amps to eliminate feedback and optimize the sound.

The boys had a split-feed going off the SBD. This, of course, was common practice (feed A goes to the PA system, feed B goes to the monitors, feed C goes to the Betty Board). The PA feed is getting delayed as it goes through each device. There are equalizers, mixers, limiters, crossovers, amps, and the PA system. Sound boards have effects "send" and "return" feeds. Each device adds a slight time delay to the electronic signal. The total signal delay inside Harding Theatre is more than the signal delay of the raw unequalized audio signal mix being fed to a live shot that is microwaved directly to the radio station's FM transmitter.

There is only one way that a delay like what they describe could be noticed by anyone. You can see it with an audio signal test generator being fed into the system. Then you can measure the time differential on a 2-channel oscilloscope with the 2 different signals from the split-feed. The SBD mix fed to your FM radio was a few hundred microseconds faster than the SBD mix fed to the PA to the crowd. Jerry and Bob were discussing this.

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Poster: johnnyonthespot Date: Sep 5, 2010 12:27pm
Forum: GratefulDead Subject: Re: Edwin H. Armstrong and The Grateful Dead

Thanks Monte! and by the way that is one of my all time favorites. I especially love the tone Phil used to get. I don't care what the fans of five and six strings bass say, he got more to my ears out of those fours strings.

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Poster: laptap3r Date: Feb 2, 2011 3:21pm
Forum: GratefulDead Subject: Re: Edwin H. Armstrong and The Grateful Dead

I think a simpler, more likely explanation of the delay Jer and Bob were talking about was "light is faster than sound". The radio listeners were mostly closer to their speakers than the front row audience was to the PA stack, and the time it took the signal to travel from instrument to radio tuner was negligible. For perspective, in the same time a sound wave travels one foot from a speaker, an FM signal will travel 186 miles from the tower. That meant that even though it took a fraction of a second more for the signal to travel to the individual radio set than it took to travel to the PA stack, it took a much longer fraction of a second (up to half of one in the larger venues) for the sound wave to travel from the PA stack to the listeners' ears than it did to travel from home stereo speakers to a home listener's ears (most concertgoers would be more than fifteen feet from the stacks, up to three hundred, at a millisecond per foot - a home listener would be anywhere from five feet to no more than fifty).

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Poster: dead-head_Monte Date: Feb 5, 2011 9:07am
Forum: GratefulDead Subject: Re: Edwin H. Armstrong and The Grateful Dead

Thanks for your comments. This is an interesting scenario. Here's the explanation from my point of view.

speed of light is much faster: 186,282 miles per second
speed of sound is way slower: 768 miles per hour

I visualized an experiment in my mind when I commented about what Jer was saying. Furthermore, I tried to imagine what some of the circumstances would have been. Why would Jerry ever say something so unusual? Then I asked myself, what conversations may have been going on -- between the FM radio engineers, GD sound crew, and the band -- while they were setting up and fine tuning all their equipment?

I believe that Jerry mentioning the "radio [path] audio faster than the PA [path] audio" was because GD were impressed and inspired earlier that day with conversations the engineers were having with them. Perhaps they had shared troubleshooting and fixing some problems while they were setting everything up. Then I thought about it from there.

My mind's experiment is this:

a) Have a FM radio receiver monitor system (setup inside Harding Theatre) for their Radio path audio -- Put a mic 25 feet from these speakers. Feed this signal into Channel 1 on your oscilloscope.

b) For the Harding Theatre path PA audio -- Put a mic 25 feet from the PA speakers there. Feed this signal into Channel 2 on your oscilloscope.

Run a 1 Khz (1000 cycles) test signal from an Audio Signal Generator into channel 1 on GD's soundboard. The SBD's audio tone test signal outputs into the split-feeds. Measure these 2 signals on an oscilloscope. What do you see on the oscilloscope?

Externally lock the scope's time base reference by using the Audio Signal Generator's signal as an external reference. Set up your oscilloscope's time base by using Delayed Sweep mode. Maximize the scope's vertical amplifiers for best presentation on the scope's CRT display. Using this method, you will be able to measure exactly how much EACH path's delay is from the source - with respect to the Audio Signal Generator source. Then measure the difference. This is how I answered John's question.

The FM Radio live-shot had fewer electronic devices to delay signals in the A path. The PA System had lots of electronic devices to delay signals in the B path. Mathematically, sum together the delay totals for each path. This is my experiment's thesis. I find that the A path is faster (arrives sooner).

800px-Tektronix_465_Oscilloscope.jpg



This post was modified by dead-head_Monte on 2011-02-05 17:07:08